Several aspects to consider in the design of desulfurization schemes for methanol plants

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Several aspects to consider in the design of desulfurization schemes for methanol plants. China is a country with abundant coal resources but relatively scarce oil resources. With the rapid economic development in China and the continuous rise in international oil prices, China’s energy industry that relies on coal as a raw material has developed swiftly. In recent years, amid rising gasoline prices, methanol plants that use coal as a raw material have seen rapid growth in China, with their scale increasing from tens of thousands of tons to hundreds of thousands of tons. 0wv2H Y}#C#P| In the early days, methanol production facilities in our country mainly relied on the combined alcohol process; as market demand continued to grow, more facilities using the single-alcohol process were built. Meanwhile, the design of these methanol production facilities also became increasingly refined and mature. Based on my experience in the construction of methanol facilities, I will discuss several aspects that should be taken into consideration when designing methanol desulfurization plants that use coal as a raw material. 9kA h M0x – Reduce desulfurization of water gas, and increase desulfurization after conversion. 7j H'\)i)A k,}p In current methanol production plants using the mono-alcohol route, wet desulfurization is primarily employed for both water gas desulfurization and desulfurization after conversion, due to the high sulfur content in the gases. The conversion process uses a full-gas-volume partial conversion process with low-temperature converters, and sulfur-resistant conversion catalysts are used in these converters. Such catalysts require that the sulfur content in the gas be at least a certain level; depending on the requirements of different catalyst manufacturers, this level ranges from 80 to 150 mg/Nm3. The catalyst must be heated and vulcanized before use. If the sulfur content in the gas is low during operation, it can easily lead to re-vulcanization of the catalyst, affecting its performance and the parameters of the conversion process. For desulfurization, the main goal is to remove sulfur from the gas in order to prevent high sulfur levels from affecting the synthesis catalysts; therefore, it is desirable for the sulfur content in the gas after desulfurization to be as low as possible. However, if the sulfur content in the gas after water-gas desulfurization is too low, it can lead to resulfurization of the shift catalysts. There is thus a certain conflict between these two design requirements. Based on this, the author suggests that the desulfurization capacity of water gas before the shift reaction should be reduced appropriately in the design, or even eliminated (the author has discussed the elimination of this water-gas desulfurization step in another paper). Additionally, an extra bypass line should be added at the inlet and outlet of the water-gas desulfurization tower to better control the sulfur content in the gas after desulfurization. The desulfurization after transformation should enhance its desulfurization capacity. Regarding enhanced desulfurization after the transformation, although the investment increases slightly, it reduces the load on the precision desulfurization process, extends the service life of the precision desulfurizing agents, and still meets the requirements for the synthetic catalyst. Overall, the benefits outweigh the drawbacks. Taking a 100,000 tons per year methanol plant as an example, since many such plants are currently under construction or have already been built, the diameter of the desulfurization towers used in their water-gas desulfurization units often exceeds 4.5 meters. In addition, due to the high sulfur content in the raw coal, two-stage desulfurization is employed in these units; although this improves the desulfurization efficiency, it results in a lower sulfur content in the gas, which in turn affects the operating conditions of the shift catalysts ； At the same time, the desulfurization tower is designed to be large in size; to ensure an adequate spraying density, the flow rate of the associated circulation pumps must be increased, which raises operating costs. By reducing the water-gas desulfurization capacity, increasing the desulfurization efficiency after conversion, or even implementing two-stage desulfurization in the conversion section, it is possible to not only meet the requirements of conversion and achieve the desired desulfurization level but also adjust the intensity of the desulfurization equipment based on the sulfur content of the feed coal. This approach helps to reduce operating costs, meet process specifications, and ensure the continuous and stable operation of the plant. )X8D-w+o3p+bq g&d8PK ad6W Separation of desulfurization for water-gas and gas after shift reaction X]gh
oP For methanol and synthetic ammonia plants, rough desulfurization is generally carried out using wet desulfurization methods; wet desulfurization mainly involves desulfurizing water-gas as well as the gas produced after shift reaction, and essentially the same desulfurization technique is used in both cases. In previous designs, since the same desulfurization method was used for both desulfurization processes, some manufacturers shared a set of equipment required for desulfurization; that is, the normal desulfurization and variable desulfurization processes shared the same lean liquid tank, rich liquid tank, and spray regeneration tank. The main advantages of this design are as follows: It saves on investment and space usage, which is quite obvious – by sharing equipment, the number of devices required is reduced, thereby cutting down on investment in such equipment and also decreasing the amount of space needed. ZSeveral aspects to consider in the design of desulfurization schemes for methanol plants. China is a country with abundant coal resources but relatively scarce oil resources. With the rapid economic development in China and the continuous rise in international oil prices, China’s energy industry that relies on coal as a raw material has developed swiftly. In recent years, amid rising gasoline prices, methanol plants that use coal as a raw material have seen rapid growth in China, with their scale increasing from tens of thousands of tons to hundreds of thousands of tons. 0wv2H Y}#C#P| In the early days, methanol production facilities in our country mainly relied on the combined alcohol process; as market demand continued to grow, more facilities using the single-alcohol process were built. Meanwhile, the design of these methanol production facilities also became increasingly refined and mature. Based on my experience in the construction of methanol facilities, I will discuss several aspects that should be taken into consideration when designing methanol desulfurization plants that use coal as a raw material. 9kA h M0x – Reduce desulfurization of water gas, and increase desulfurization after conversion. 7j H'\)i)A k,}p In current methanol production plants using the mono-alcohol route, wet desulfurization is primarily employed for both water gas desulfurization and desulfurization after conversion, due to the high sulfur content in the gases. The conversion process uses a full-gas-volume partial conversion process with low-temperature converters, and sulfur-resistant conversion catalysts are used in these converters. Such catalysts require that the sulfur content in the gas be at least a certain level; depending on the requirements of different catalyst manufacturers, this level ranges from 80 to 150 mg/Nm3. The catalyst must be heated and vulcanized before use. If the sulfur content in the gas is low during operation, it can easily lead to re-vulcanization of the catalyst, affecting its performance and the parameters of the conversion process. For desulfurization, the main goal is to remove sulfur from the gas in order to prevent high sulfur levels from affecting the synthesis catalysts; therefore, it is desirable for the sulfur content in the gas after desulfurization to be as low as possible. However, if the sulfur content in the gas after water-gas desulfurization is too low, it can lead to resulfurization of the shift catalysts. There is thus a certain conflict between these two design requirements. Based on this, the author suggests that the desulfurization capacity of water gas before the shift reaction should be reduced appropriately in the design, or even eliminated (the author has discussed the elimination of this water-gas desulfurization step in another paper). Additionally, an extra bypass line should be added at the inlet and outlet of the water-gas desulfurization tower to better control the sulfur content in the gas after desulfurization. The desulfurization after transformation should enhance its desulfurization capacity. Regarding enhanced desulfurization after the transformation, although the investment increases slightly, it reduces the load on the precision desulfurization process, extends the service life of the precision desulfurizing agents, and still meets the requirements for the synthetic catalyst. Overall, the benefits outweigh the drawbacks. Taking a 100,000 tons per year methanol plant as an example, since many such plants are currently under construction or have already been built, the diameter of the desulfurization towers used in their water-gas desulfurization units often exceeds 4.5 meters. In addition, due to the high sulfur content in the raw coal, two-stage desulfurization is employed in these units; although this improves the desulfurization efficiency, it results in a lower sulfur content in the gas, which in turn affects the operating conditions of the shift catalysts ； At the same time, the desulfurization tower is designed to be large in size; to ensure an adequate spraying density, the flow rate of the associated circulation pumps must be increased, which raises operating costs. By reducing the water-gas desulfurization capacity, increasing the desulfurization efficiency after conversion, or even implementing two-stage desulfurization in the conversion section, it is possible to not only meet the requirements of conversion and achieve the desired desulfurization level but also adjust the intensity of the desulfurization equipment based on the sulfur content of the feed coal. This approach helps to reduce operating costs, meet process specifications, and ensure the continuous and stable operation of the plant. )X8D-w+o3p+bq g&d8PK ad6W Separation of desulfurization for water-gas and gas after shift reaction X]gh
oP For methanol and synthetic ammonia plants, rough desulfurization is generally carried out using wet desulfurization methods; wet desulfurization mainly involves desulfurizing water-gas as well as the gas produced after shift reaction, and essentially the same desulfurization technique is used in both cases. In previous designs, since the same desulfurization method was used for both desulfurization processes, some manufacturers shared a set of equipment required for desulfurization; that is, the normal desulfurization and variable desulfurization processes shared the same lean liquid tank, rich liquid tank, and spray regeneration tank. The main advantages of this design are as follows: It saves on investment and space usage, which is quite obvious – by sharing equipment, the number of devices required is reduced, thereby cutting down on investment in such equipment and also decreasing the amount of space needed. Cj0y 29G: The number of devices decreases, which reduces the required level of maintenance and facilitates management. However, in practical applications, this design has the following main disadvantages: -n3oaXZs${ ? During the desulfurization process, there are heat and mass transfer processes between the gas and the liquid. Since the gas contains impurities such as dust and tar, the desulfurization process is also a process of purifying the gas. However, the water gas produced by gas generation contains a lot of impurities such as dust, making the gas dirty; whereas the desulfurized water gas after conversion is cleaner with fewer impurities. The different purities of these two gases result in varying purities of the desulfurization-rich liquid from water gas desulfurization and that from shift reaction; the desulfurization-rich liquid from water gas desulfurization is dirtier. The mixing of these two liquids in the regeneration tank makes it difficult to control the regeneration of the desulfurization liquid, thereby affecting the effectiveness of regeneration. V8iH:Y~Q have different operating pressures. The operating pressure for water gas desulfurization is relatively low, typically around 30 kPa. Due to this low pressure, the operating temperature during the regeneration of the desulfurization solution is set at a level that allows for optimal regeneration. In contrast, the pressure in the gas after conversion is usually 8 or 20 kilograms per square centimeter. When this pressurized gas enters the desulfurization tower, the higher gas pressure increases the solvency of the desulfurization solution, enabling certain gases, especially carbon dioxide, to dissolve in it. To facilitate better regeneration and reuse of the desulfurization solution, its regeneration temperature should be 2–3°C higher than that used in water gas desulfurization. However, if two streams of rich liquid enter the same regeneration tank, their mutual influence poses difficulties in operation. 1L}@ fvOQ p 3 For the regeneration of the desulfurization liquid, it is necessary that the desulfurized liquid fed into the regeneration tank have a flow rate and pressure that are as stable as possible, while also having minimal fluctuations in the liquid level; this facilitates the regeneration of the desulfurization liquid and the flotation of sulfur bubbles. If the normal desulfurization process and the modified desulfurization process share the same regeneration tank, there are two streams of desulfurization fluid entering this tank. The desulfurization fluid from the modified process enters the regeneration tank directly under the pressure generated by the conversion process, while the desulfurization fluid from the water-gas desulfurization process enters the tank after being pressurized by a liquid-rich pump. Due to their different origins, these two streams have varying flow rates and head pressures; as they enter the regeneration tank simultaneously, they affect each other, making it difficult to regulate the operation process. This in turn affects the stability of the desulfurization process and hinders the regeneration of the desulfurization fluid. Given the above circumstances, desulfurization of water gas is separated from desulfurization after conversion, with a separate set of regeneration equipment for each, thereby achieving a better desulfurization effect and ensuring continuous and stable operation of the desulfurization process. III. Regarding the desulfurization arrangement after conversion 2v5~*l*g)B: During the conversion process, most of the organic sulfur in water gas is hydrolyzed to form hydrogen sulfide; as a result, the hydrogen sulfide content in the gas after conversion increases, and it is necessary to purify the gas again using a desulfurization tower to remove the sulfur present in it. Different manufacturers have varying opinions regarding the equipment layout for this desulfurization section, and design considerations can be based on the following aspects. 1. In terms of management, for the desulfurization of water gas and that of synthesized gas, most manufacturers use the same desulfurization method, which is generally wet desulfurization. Since the same desulfurization method is used, equipment such as the circulating pump for the lean liquid tank and the regeneration tank in the desulfurization tower are required, and the types of these devices are generally similar. Therefore, for ease of management, the equipment for water-gas desulfurization and desulfurization after conversion is arranged together as much as possible to facilitate unified control. 2F(N!YC3u/F Regarding process requirements and investment, as mentioned earlier, for the sake of easier management, the equipment for water-gas desulfurization and desulfurization after conversion should be placed together as much as possible. However, the conversion desulfurization tower is located after the conversion step, and there is a compression section between this equipment and the water-gas desulfurization equipment. If the regeneration tower is located in the conversion section, both the inlet and outlet pipelines for the desulfurization fluid must come from the water-gas desulfurization section, which poses difficulties in management ； If the shift converter tower is also placed in the water-gas desulfurization section, the water-gas inlet pipeline of the shift converter tower will enter the desulfurization section from the shift conversion section, while the outlet pipeline must enter the decarburization section from the water-gas desulfurization section. Due to the relatively large diameter of the gas pipelines, pipeline layout becomes difficult, resulting in increased investment. 3^
|^&N m'\X The impact of decarboxylation methods: With the continuous development of methanol production processes, more and more manufacturers are choosing pressure swing adsorption for methanol decarboxylation, owing to its many advantages such as high level of automation in the equipment, a clean working environment, and low operating costs. Pressure swing adsorption also provides excellent desulfurization effects while carrying out carbon removal. To reduce the burden associated with desulfurization, some manufacturers send the gas directly to the carbon removal stage after the reforming process, and then to the desulfurization stage after carbon removal. The advantage of this approach is that pressure swing adsorption can be used for desulfurization; since the sulfur content in the gas after reforming is low, desulfurization can be achieved simultaneously through pressure swing adsorption. As a result, the desulfurization process after carbon removal can operate without being used at all or with reduced capacity, thereby lowering operating costs ； However, the drawback is that the gas used in the decarburization and carburization stages comes directly from the shift process and contains high levels of sulfur; as a result, when pressure swing adsorption is used for desulfurization, the hydrogen sulfide content in the released air is high, leading to environmental pollution. g0Q.w]1oC 4. Treatment of gas after rough desulfurizationVlc#a\.{^2e In the water-gas desulfurization unit, after the gas emerges from the desulfurization tower, it still contains water vapor and other impurities; therefore, it is best not to send it directly to the compressor. It is advisable to carry out the following treatments before sending the gas to the compression stage. Ri#\(p&t"gv{ 1$^WaW])r – Set up the scrubber tower for washing. }T"Br'i8D$s2~4r: During the desulfurization process, a small amount of elemental sulfur enters the gas; at the same time, the gas still contains impurities such as tar. These impurities can easily reach the compression section and clog the inlet valves of the compressors, resulting in the need to shut down the compressors for cleaning and thus affecting production. Through washing, the small amount of elemental sulfur produced during desulfurization is removed, further purifying the gas and facilitating stable production. Taking our company’s ammonia synthesis plant as an example, before the installation of the gas scrubber, the compressor inlet valves would often get clogged, requiring cleaning 3-4 times a month. After the addition of the gas scrubber through modifications, the interval between valve cleanings has extended to over 3 months. H3b.ez U: Depending on the properties of gases, the lower the temperature, the smaller the volume. Therefore, after the gas is washed, its temperature drops and its volume decreases, which in turn increases the amount of air pumped by the compressor and boosts production capacity. 2. Installation of dust collectors: Since water-gas is a flammable and explosive gas, any equipment through which it passes must have its interior replaced before maintenance can be carried out, which makes maintenance more difficult. It is recommended to install two electrostatic dust collectors in the desulfurization unit; this not only improves the purity of the gas but also ensures that even if one collector fails, it will have little impact on the operation of the facility for a short time. 3D7FT L_M1O#}pX 3: Set up a separator. Hydrogen sulfide, in the presence of water, can be easily electrolyzed to produce hydrogen ions and hydrogensulfate ions; these substances are corrosive to pipes and equipment. The gas coming out of the desulfurization tower contains a certain amount of water vapor and hydrogen sulfide, and it is necessary to remove most of this water using a separator before the gas can proceed to the compression stage. An9`k4F"Ed 5 Regarding the desulfurization circulating water: Since the cooling towers in the desulfurization unit require cooling water, and the gases produced in this unit are still quite dirty, if the circulating water from this unit is integrated into the circulating water system of the entire methanol plant, it can easily contaminate that system and have an adverse effect on it ； If it is integrated into the circulating water system of the gas generation unit, the high levels of impurities such as dust and tar in the gas generation circulating water can negatively affect the quality of the desulfurization circulating water. Therefore, given these circumstances, it is recommended that manufacturers with the necessary resources establish a separate circulating water system for desulfurization, thereby ensuring the continuous and stable operation of the entire facility. M$b3m u#U 6 Regarding sulfur recovery}I"?-}g"a9Y}0? As for sulfur recovery, due to its relatively simple process and low operational requirements, some manufacturers do not pay enough attention to this aspect. In fact, there is still much work that can be done, from design to management; it is highly beneficial for improving energy efficiency, reducing consumption, and ensuring continuous stable operation of the equipment. Here, only a few points will be discussed. 1Vxb ]yo{,F(Hn Selection of sulfur melting reactors and arrangement of sulfur foam tanks. Currently, there are two types of equipment for sulfur recovery: batch sulfur melting systems and continuous sulfur melting reactors. For intermittent sulfur melting, the equipment capacity is limited; a methanol plant with an annual production capacity of 100,000 tons requires two sulfur melting reactors, and in some cases additional equipment such as surge tanks is also needed. This results in large space requirements for the equipment. In contrast, continuous sulfur melting reactors have a higher capacity – a methanol plant with an annual production capacity of 100,000 tons only needs one such reactor. Such reactors require less space, are easier to operate, and can be installed outdoors, which represents certain advantages. Due to the high viscosity of sulfur foam, the pipeline leading from the regeneration tank to the sulfur foam tank must have a certain slope. Moreover, these two units should be placed as close to each other as possible to reduce the length of the pipeline required for transportation and prevent blockages in the pipes. If necessary, cleaning ports should be installed at areas where blockages are likely to occur, to facilitate cleaning. 2-W Uq M hG W3l\w Pay attention to the recycling of desulfurization residues. During the sulfur melting process, a certain amount of desulfurization liquid is discharged from the sulfur melting vessel. This liquid has a high temperature, around 150°C, and its catalyst content is 4–5 times that of the concentrated liquid. Therefore, if this liquid is discarded directly, it not only causes environmental pollution but also results in a loss of catalysts; it must be recycled effectively. Based on the actual conditions in some manufacturers, if this clear liquid is used directly in the regeneration tank, its high temperature and high catalyst content can easily disrupt the balance within the tank, making it difficult to produce and float elemental sulfur, thus affecting the continuity and stability of the desulfurization process. It seems that a good approach at present is to install 2-3 storage tanks, where the desulfurization effluent undergoes processes such as cooling and sedimentation within these tanks before being sent to the lean liquid tank for reuse. This requires that the treatment conditions and requirements for the clear liquid be fully considered during the design phase. 3 Other aspects: For sulfur foam pumps, the pump’s head and flow rate must meet the requirements of the sulfur melting tank; moreover, since the medium being transported contains a certain amount of particles, it is advisable to use a packing seal for such equipment. The sulfur recovery process is somewhat unstable and intermittent; as a result, the pipelines used for transporting sulfur foam are prone to corrosion. Manufacturers with the necessary resources can opt for pipelines made of stainless steel. 7y:O/o/tWw_;?V b4_1dz-U,kk$JE Seven Other aspects: To facilitate replacement and ensure safe operation, manufacturers with the necessary resources can install nitrogen pipelines near the equipment; these pipelines can be connected using hoses when needed. To ensure stable operation, automatic control is preferably used for the parameters that need adjustment, thereby increasing the level of automation of the equipment, reducing the number of operators, and lowering operating costs. To facilitate the installation of equipment and processes as well as the operation and maintenance of installations, comprehensive consideration and thorough evaluation must be given to equipment layout and process piping. x;C/kJ%d D$F 1b@%{2\@/gi\8B Eight Summary The above are some of the author’s insights gained during the construction of methanol plants. Different manufacturers need to take into account various factors such as their own raw material conditions, existing utility facilities, product requirements, and safety aspects, in order to determine the most suitable process scheme for their own plant.